It is known to provide so-called packed towers for the washing of gases and, in general, tower-like apparatus for this purpose can use many of the mass-exchange principles known in the art.
In a packed (trickle) tower, the gas is passed upwardly through the interstices of a packing while the washing liquid is sprayed into the gas within the packing or thereabove, the gas undergoing many direction changes within the body of the packing material.
Such systems are characterized by a high pressure drop and are not always fully effective.
In other scrubbing systems, plate-type or impingement baffles are provided which cause deposition of the dust-entrapping liquid phase upon the surfaces which lie transversely to the gas stream intercept the latter. In some of these arrangements, a high efficiency of liquid recovery can be obtained while others are characterized by less efficient operation.
When gaseous components of the gas stream are to be solubilized or washed out, in addition to or apart from dust removal, still other systems may be provided to obtain maximum intimacy of contact between gas and the liquid phases.
With increasing attention to environmental pollution and particularly, greater efforts to remove dust from industrial gases before they are released into the atmosphere to eliminate toxic and nauseous gas components, and to release large volumes of relatively clean and uncontaminated gas into the atmosphere, considerable research has been undertaken to improve upon gas-washing and scrubbing systems of all types. It may be said, in this connection, that none of the prior art systems have been able to remove particulates and soluble gas components from a gas stream with low energy expenditure, small pressure drop, high efficiency and low apparatus costs heretofore.
More particularly, the conventional drip-type separators also known as wire mesh separators, may pass the gas through a stack of wire gauze, wire cloth or the like, this material constituting "packing" of a tower in the manner described. These systems have poor separating efficiency and are characterized by considerable pressure drop.
Of perhaps greater importance is the disadvantage that such systems must operate with a narrow range of operating parameters if even the limited efficiency of the system is to be maintained. Accordingly, significant deviations from the optimum operating parameter results in serious drops in efficiency and thus the systems have limited applicability. In such systems, as in all packed-tower systems, there is a tendency for solid deposits to form on the wire mesh and obscure the flow passages, thereby further increasing the pressure drop and requiring frequent and expensive maintenance.
Impingement or rebound towers have been used for recovering droplets of water entrained in a gas, particularly downstream of the cooling towers of power plants and other industrial installation. These systems have been found to be practical only where the gas enters the tower at a relatively high velocity. These systems are moreover, also characterized by the disadvantage that they operate only with a relatively narrow range of parameters before the efficiency falls off to a technologically unaccessible level.